Optical beamsplitters are used in a wide variety of optical apparatus. Typical beamsplitters include a surface which reflects a portion of incident light and transmits much of the remaining light. In most cases, it is desirable to have the light reflected be a fixed fraction of the light input or of the light transmitted, and this fraction should be reasonably independent of the polarization and wavelength of the input light. These conditions are particularly important when using a beam splitter to monitor optical power. Unfortunately, light reflected from the two major surfaces can interfere and produce an undesired ripple in the wavelength response of the beamsplitter. Further, when the beamsplitter is at an angle to the incident light, different polarizations will be reflected with different strengths producing polarization dependent loss (PDL).
Beamsplitters have been proposed which are independent of the polarization of the input light. For example, U.S. Pat. No. 4,492,436 issued to Bergmann describes a beamsplitter including a slab of birefringent material which changes the polarization of light refracted from the first major surface relative to the light reflected from the first surface. When the refracted light is reflected from the second major surface that reflection will be polarized orthogonally to the light reflected from the first major surface and no significant interference between the reflections of the two surfaces will occur.